Southern Discomfort

In AR4 (the 4th assessment report of the Intergovernmental Panel on Climate Change) the trend in Antarctic (southern hemisphere) sea ice was reported as small (5.6 +/- 9.2 thousand km^2/yr) and not statistically significant, but in AR5 (the 5th assessment report) it is reported as both statistically significant and much larger (16.5 +/- 3.5 thousand km^2/yr). Even at this rate the Arctic is still losing sea ice 3 times as fast as the Antarctic is gaining it, but the larger trend is still surprising; such a large rate of increase is, more and more, turning out to be incompatible with computer model simulations.

It turns out that the difference between AR4 and AR5 results was not just because of the addition of a few extra years’ data, it was also (in fact primarily) because the data set itself was revised. The latest is called “version 2” and was published in 2008 by Comiso and Nishio (Comiso, J. C. and Nishio, F., 2008. Trends in the sea ice cover using enhanced and compatible AMSR-E, SSM/I, and SMMR data, J. Geophys. Res., 113, C02S07, doi:10.1029/2007JC004257). They reported at the time that it made little difference to the trend in Antarctic sea ice. But comparison with the trend reported in AR4, and deduced from a “version 1” data set, show that this is not the case. The conclusion is that there was another revision, to version 1 only, which came after the research on which AR4 based its results but before Comiso and Nishio implemented the revision which brought us to version 2.

Eisenman et al. directly compared the available data for version 1 (the pre-revision data) and version 2, plotting the difference (v2 minus v1) in their figure 2:

The “step change” nature of the difference is visually evident — quite strikingly so — and easily confirmed statistically. Clearly there is a difference between data before and after December 1991 which is due to the change in the way the raw data were processed. Perhaps most suggestive is that the step occurs at precisely one of the moments when there was a transition between different sensors (indicated by the vertical dotted lines). This is too implausible to be considered a coincidence. They point out that

… subtracting 0.15×10^6 km2 from Version 2 in all months after December 1991 causes the trend to be nearly equivalent to Version 1 for the range of endpoints plotted in Fig. 1b (not shown). Hence the issue appears to be associated with erroneous calibration across the December 1991 sensor change in one of the two Bootstrap versions.

The big question is, as they say, “whether the undocumented change from Version 1 to Version 2 introduced an error or removed one,” and they try to determine which is the case but none of their methods resolves the issue unequivocally. They do speculate, pointing out that if v1 is correct then there is less growth of Antarctic sea ice and therefore greater agreement with climate models, which favors v1 being more correct.

I don’t find that argument particularly persuasive. I’m not one to bash climate models, but as far as I know, the change in sea ice is one of their weaknesses, not their strenghts.

I have, however, found what may be evidence to support their contention. I took the southern hemisphere sea ice extent from NSIDC (Nation Snow and Ice Data Center) which uses the version 2 bootstrap algorithm they studied, computed anomaly (to remove the seasonal cycle), and smoothed it on a short time scale (about 5 years). I got this:

Note that there appears to be a step change at about the time of the step change in difference between v1 and v2, and this is derived from studying the v2 data all by itself.

The step change in the smooth looks to be closer to 1994 than 1992, but if there were an actual dip around 1993, that’s exactly what we would see. I even took the v2 data and subtracted 0.2 million km^2 from all data after December 1991 (even more than the 0.15 million km^2 subtracted by Eisenman et al.) to see whether this “adjusted” data (with even larger adjustment than they suggest) would throw things out of whack. It did the opposite, making things more consistent and constant:

I consider the time evolution shown in the adjusted data to be more plausible than the unadjusted data. This suggests that it’s more likely the error was introduced in the undocumented revision just prior to v2, so there is still an erroneous transition between sensors around December 1991.

On a different topic entirely: if you’re reading this but not at its blog of origin (tamino.wordpress.com), and at the top is a line saying “Written by Amego Sando,” then you should be aware that that is a lie. This post was NOT written by Amego Sando, he’s just a thief who steals other people’s blog posts and re-posts them while claiming credit for authorship. Oh — and if you actually happen to be Amego Sando and you’re reading this: va te faire mettre.

I do take exception with one statement in the Eisenman et al. paper. As early as the abstract they state:

The results of this analysis raise the possibility that this expansion may be a spurious artifact of an error in the satellite observations, and that the actual Antarctic sea ice cover may not be expanding at all.

Note: they don’t claim that there’s no increase in Antarctic sea ice, they only raise the possibility. But I don’t consider that realistic at all. If I apply the 0.2 million km^2 adjustment, an even bigger “correction” (if it is one) than they suggest, it reduces the trend but doesn’t eliminate it. Furthermore, the increase is still statistically significant. Using the v2 data without adjustment I get a linear trend rate of 17.6 +/- 8.0 thousand km^2/yr, when the even-bigger adjustment is applied the rate decreases to 9.7 +/- 4.7 thousand km^2/yr — far less growth, but still statistically significant.

Bottom line: it seems to me that the evidence shows convincingly there is a discontinuity in at least one of the data sets at the moment of one of the sensor transitions. There is the distinct possibility, maybe even a “more likely than not,” that the data currently favored (v2) has a spurious increase which exaggerates the apparent trend. But even if that is the case, Antarctic sea ice still shows statistically significant increase since satellite observations began. And it’s well not to forget that however much sea ice the Antarctic may have increased, the Arctic has lost a whole lot more sea ice than the Antarctic has gained.

When there is a discrepancy between data and models, it is interesting how often some people assume there is a problem with the fundamental underlying theory (e.g., the greenhouse effect) rather than the data (e.g., systematic errors) or the implementation of the theory (e.g., spatial resolution in a computer model). This is an odd, given problems with data and implementation of theory are more common than a failure of the fundamental underlying theory.

On a related topic, @fluids_guru on twitter has noted that radio occultation measurements suggest that some of the discrepancies between models and temperature data at some altitudes/latitudes may (in part) be the result of errors in some datasets (http://www.atmos-meas-tech.net/4/1965/2011/amt-4-1965-2011.pdf).

I don’t want to blame anyone without evidence so please view this as a question out of curiosity not as an accusation nor as a hint or suggestion of what will or might happen. Could new data from introduction of 2008 sensor cause a recalibration resulting in such large changes? For example, if Comiso and Nishio (2008) should have said something like ‘the effect of change in the algorithm is negligible but large differences between sensors readings have caused a large recalibration resulting in a significant change in the trend’, would this be consistent with what is shown in the paper? i.e. there wasn’t an error in V1 or V2 just new information?

[Response: Not really. Regardless which is correct, the difference between v1 and v2 is only consistent with an erroneous calibration across a sensor transition (unless it’s the handiwork of Harry Potter).]

Even if you’re not using the same data, it will give the authors some idea of test they can perform to make their paper stronger. I know you’re probably strapped for time, but if it were my paper, I would appreciate the comment.

[Response: You’re right. They may indeed have identified a necessary correction, but from what I see the upward trend in southern sea ice is still robust.]

That is, the CIs of the trend lines do not overlap at the 95% level while the trend slopes themselves are not significantly different. (graph is at: http://www.nfgarland.ca/Rplot02.pdf)

[Response: Fascinating. Perhaps you too should submit a comment to the discussion at “the Cryosphere.” One caveat: is there autocorrelation correction to the trend uncertainty? BUT — even if not, your point is relevant and worth sharing with the authors.]

The point that immediately leapt out to me was that the .178 million areal difference here was very much in the range your .200 value and the paper’s .150 value.

No autocorrelation correction on the CIs other than subtracting out monthly means across the entire 35 year period.

[Response: I submitted a comment to the discussion at “TC” and noticed that Kevin Cowtan has pondered essentially the same question. I reiterate my suggestion that you share your results in the comments there.]

Would an increase in fresh meltwater (floating on top of denser salt water) lead to an increase in ice (presumably area detected from satellites)? I’d guess fresh water freezes more readily. Pure speculation on my part.

Hank Roberts – the Manabe et al 1991 paper Dumb Scientist pointed to discusses this; according to their modeling increased precipitation from warming freshens the surface waters, increasing the halocline gradient (salty below, fresh above) and thus reducing exchange. The surface waters are then less connected to the warmer deep waters, a smaller thermal mass, and more ice occurs. I’m sure increased meltwater doesn’t help either, but I don’t believe that was part of the Manabe model.

A counter-intuitive result, and certainly not one of the outputs of most climate models.

I wonder if it’s possible to convert the sea ice area measurements into some estimate of net change in heat absorbed at the surface. We commonly point out that winter Antarctic sea ice doesn’t have much light to reflect in the first place, while summer Arctic ocean can soak up lots of heat. I wonder how detailed you’d need to be to get a first pass at this.

If you know what you’re doing, this is probably trivial…if not it would be laborious to sort out.

Another issue that I haven’t seen discussed is the effect on outward radiation budget from the increase in wintertime sea ice. I downloaded Antarctic Sea ice cover from woodfortrees and the wintertime max area is increasing ~2X the summer min (OLS linear trends, winter +2.0e-2 R^2=0.26, summer +1.1e-2 R^2=0.085). As I pointed out on another blog, “The ocean surface is constrained to be at or above the seawater freezing point, roughly 270 Kelvin, but the ice insulates the surface from the ocean heat below, allowing the surface temperature to drop to -30 to -40 degrees C – about 240 Kelvin. Due to the fourth power dependency of thermal emission versus temperature, 270000 km^2* of ice at 240 K radiates about 3e13 less watts than the seawater underneath would radiate at 270 K. In 4 months in the dead of one winter, this would amount to 3e20 extra joules in earth’s climate system, enough to melt 10^12 cubic meters of ice.
*this is approximately the accumulated difference between 1980-85 and 2008-13.

Neven’s sea ice blog discussed this quite a bit in the Arctic context–where it is, of course, a negative feedback to warming rather than a positive one, as in the Antarctic case. Unfortunately, I don’t recall if any actual papers came up in the discussion.

“this expansion may be a spurious artifact of an error in the satellite observations, and that the actual Antarctic sea ice cover may not be expanding at all.”

But most of the expansion appears to have happened after 1991 (regardless which dataset you look at), and a spurious step change in 1991 should not affect the post 1991 trend. Using only 10 years of bootstrap data from 1992-2012 (i.e. after the putative step increase) yields a positive trend similar to the AR5 value (at least using simple OLS without accounting for autocorrelation), so the trend can’t be explained solely by a spurious addition of a constant to all post 1991 values (unless autocorrelation makes a huge difference to these analyses). A positive trend post 1991 could only be explained by observation error if there was a trend in the errors – the paper presents no evidence of that does it?

Tamino, I was looking over your 2012 post on insolation that one of your other readers guided me to.

I was immediately struck by the graph (figure 2) and thought “It’s upside down”. I take insolation to be a positive number- the more sunlight that is absorbed the greater the insolation.

Heat absorbed by the North Pole should be going UP as a consequence of less sea ice, not down, and the reverse for the South. I’m wondering if somehow something got reversed in your calculations…. or if someone will set me straight on what’s up and what’s down in insolation.

So the absolute value of the heat differential is right…but the signs of trends in both hemispheres are reversed?

[Response: What’s plotted is the insolation on the *ice* — and as ice declines, it gets less insolation so the open ocean gets more.]

Tamino, thanks for the explanation. Just my opinion, but the total insolation of the poles would be more intuitive to explaining this to people. For one, people think of ice as being reflective…so insolation to the ice isn’t as important as insolation to the open ocean. That is the net insolation for the north pole= ice (declining)+ open ocean (increasing). Obviously covering the ice with carbon black changes things…but I think you see my point. I think you’d find also that the number for increasing insolation in the Northern Hemisphere vs decreasing insolation in the southern hemisphere would be a bigger, more dramatic number.

Or are you approximating that the loss of insolation to sea ice is close enough ocean gain? I wouldn’t have thought so, I would have guessed about a 30% or more increase.

[Response: The decrease in sea ice insolation *is* equal to the increase in ocean insolation. Remember that insolation is the *incident* radiation, not the absorbed energy (which is what drives climate). Later in the post I estimate how much climate forcing changes due to this shift of insolation from ice to ocean.]

explained how we should expect that continued global warming will actually cause increasing Antarctic sea ice for perhaps even a few more decades until it finally reverses with continued global warming.

Here is a quote from the above Science Daily report:

” “We wanted to understand this apparent paradox so that we can better understand what might happen to the Antarctic sea ice in the coming century with increased greenhouse warming,” said Jiping Liu, a research scientist in Georgia Tech’s School of Earth and Atmospheric Sciences. ”

So how could the global warming deniers point to increasing Antarctic sea ice as their alleged evidence against global warming when even one of the scientists they claim may be on their side actually demonstrates that and how they cannot use this increase as their alleged evidence?

Looks like there is an indirect effect (same principle as that suggested by Manabe and others, but with a different source of fresh water).

Here we show that accelerated basal melting of Antarctic ice shelves is likely to have contributed significantly to sea-ice expansion. Specifically, we present observations indicating that melt water from Antarctica’s ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves. Simulating these processes in a coupled climate model we find that cool and fresh surface water from ice-shelf melt indeed leads to expanding sea ice in austral autumn and winter. This powerful negative feedback counteracts Southern Hemispheric atmospheric warming. Although changes in atmospheric dynamics most likely govern regional sea-ice trends4, our analyses indicate that the overall sea-ice trend is dominated by increased ice-shelf melt.

Any suggestions on where to get a (layperson’s) summary of the bigger picture for Antarctic ice? Land and sea, sheets and shelves!

I want to know how much land ice volume/mass is changing to give some perspective to the winter sea ice growth. I recall ice shelf breakups (tend to be newsworthy) but not ice shelf recoveries (are there any?). Sea ice stats tends to be focused on area, not volume or mass making easy comparisons difficult – something hard enough for the stat’s constrained layperson to do.

In the climate science vs denier debate there is an unfortunate tendency to debate within the bounds the deniers have set (Antarctic sea ice in isolation rather than all ice or short term variations in average surface air temperatures rather than the entire globes’ heat content over a significant period).

Narrow focus is the friend of the climate science denier; I think we need to broaden it at every opportunity even if it can look like a reverse gish gallop to shift the discussion from sea ice to ice sheets or from surface air temperatures to ocean heat content.

Hank, I was just thinking that if you compared the mass of ice lost off the continent it may be a similar magnitude to the mass of sea-ice ‘gained’ over time. Not all are large bergs, and most ice loss happens in summer. This thicker ice would linger as well, compared to the thinner sea ice. I haven’t attempted to do any of the maths, maybe I will compare the values myself. I am not sure if the measured sea ice area does not include any land ice.

I don’t think the idea works, Nathan. As you say, “most ice loss happens in summer,” yet the increase in sea ice area is greater for winter than for summer. Moreover, most Antarctic sea ice melts out in summer (or, to put it another way, there’s a big annual swing for extent and area in the Antarctic–summer extent is on the order of 3 million square kilometers, while winter extent is about 18 million.) That strongly suggests that we are looking at true melt/freeze cycles, not exports from land.